Method and structure for forming high-k gates

ABSTRACT

A method for forming an improved gate stack structure having improved electrical properties in a gate structure forming process A method for forming a high dielectric constant gate structure including providing a silicon substrate comprising exposed surface portions; forming an interfacial layer over the exposed surface portions having a thickness of less than about 10 Angstroms; forming a high dielectric constant metal oxide layer over the interfacial layer having a dielectric constant of greater than about 10; forming a barrier layer over the high dielectric constant metal oxide layer; forming an electrode layer over the barrier layer; and, etching according to an etching pattern through a thickness of the electrode layer, barrier layer, high dielectric constant material layer, and the interfacial layer to form a high dielectric constant gate structure.

FIELD OF THE INVENTION

The present invention relates generally to CMOS device fabricationprocesses and, more particularly, to a structure and method ofmanufacturing high dielectric constant gate structures for improvedintegration with polysilicon gate electrodes to achieve improvedelectrical properties.

BACKGROUND OF THE INVENTION

Fabrication of a metal-oxide-semiconductor (MOS) integrated circuitinvolves numerous processing steps. A gate dielectric, typically formedfrom silicon dioxide, is formed on a semiconductor substrate which isdoped with either n-type or p-type impurities. For each MOS field effecttransistor (MOSFET) being formed, a gate electrode is formed over thegate dielectric, and dopant impurities are introduced into the substrateto form source and drain regions. A pervasive trend in modern integratedcircuit manufacture is to produce transistors having feature sizes assmall as possible. Many modern day semiconductor microelectronicfabrication processes form features having less than 0.25 criticaldimensions, for example in future processes even less than 0.13 microns.As feature size decreases, the size of the resulting transistor as wellas transistor features also decrease. Fabrication of smaller transistorsallows more transistors to be placed on a single monolithic substrate,thereby allowing relatively large circuit systems to be incorporated ona single die area.

In semiconductor microelectronic device fabrication, polysilicon andsilicon dioxide (SiO₂) are commonly used to respectively form gateelectrodes and gate dielectrics for metal-oxide-semiconductor (MOS)transistors. As device dimensions have continued to scale down, thethickness of the SiO₂ gate dielectric layer has also decreased tomaintain the same capacitance between the gate and channel regions. Athickness of the gate oxide layer of less than 2 nanometers (nm) will berequired to meet smaller device design contraints. A problem with usingSiO₂ as the gate dielectric is that thin SiO₂ oxide films may break downwhen subjected to electric fields expected in some operatingenvironments, particularly for gate oxides less than about 50 Angstromsthick. In addition, electrons more readily pass through an insulatinggate dielectric as it gets thinner due to what is frequently referred toas the quantum mechanical tunneling effect. In this manner, a tunnelingcurrent, produces a leakage current passing through the gate dielectricbetween the semiconductor substrate and the gate electrode, increasinglyadversely affecting the operability of the device. Another increasingproblem is the formation of trapping states and interfacial chargedstates at the silicon/gate dielectric interface which increasinglyadversely affects device electrical characteristics. For example, as thetrapped charges accumulate over time, the threshold voltage V_(T) mayshift from its design specification. Dielectric breakdown of thinnergate dielectrics is also likely to occur at lower values of applied gatevoltage in part due to an increase in the relative volume of defects inthe gate dielectric. Such defects have an increasingly adverse effect asthe gate dielectric gets thinner. For example, a thin gate oxide oftenincludes defects due to uneven growth of the gate oxide due to defectsat the silicon surface.

Because of high direct tunneling currents, SiO₂ films thinner than 1.5nm cannot be used as the gate dielectric in CMOS devices. There arecurrently intense efforts to replace SiO₂ with high-k (high dielectricconstant) dielectrics, including for example, TiO₂, Ta₂O₅, ZrO₂, Y₂O₃,La₂O₅, HfO₂, and their aluminates and silicates attracting the greatestattention. A higher dielectric constant gate dielectric allows a thickergate dielectric to be formed which dramatically reduces tunnelingcurrent and consequently gate leakage current, thereby overcoming asevere limitation in the use of SiO₂ as the gate dielectric. Whilesilicon dioxide (SiO₂) has a dielectric constant of approximately 4,other candidate high-k dielectrics have significantly higher dielectricconstant values of, for example, 20 or more. Using a high-k material fora gate dielectric allows a high capacitance to be achieved even with arelatively thick dielectric. Typical candidate high-k dielectric gateoxide materials have high dielectric constant in the range of about 20to 40.

There have been, however, difficulties in forming high-k gate oxidedielectrics to achieve acceptable processing integration between thehigh-k gate dielectric and the polysilicon electrode formed overlyingthe high-k gate dielectric. For example, post deposition annealingtreatments of high-k dielectric layers in the presence of oxygen havebeen found to detrimentally affect high-k dielectric films by leading tocrystallization of the film and formation of an interfacial SiO₂ layerduring the annealing treatment. SiO₂ interfacial layer formation atmaterial layer interfaces creates a low dielectric constant layer inseries with the high dielectric layer and therefore reduces theeffective capacitance of the stacked layers.

Another problem associated with the above-mentioned high-k dielectricsis that the forming of a crystalline structure under normal preparationconditions leads to a roughened film surface. Surface roughness causesnon-uniform electrical fields in the channel region adjacent thedielectric film. Such films are not suitable for the gate dielectrics ofMOSFET devices.

Other processing difficulties with high-k dielectric materials includethe tendency that high-k dielectric materials are relatively difficultto etch, unlike a conventional thermally grown oxide. Chemical etchantsused with high-k materials may cause damage to associated oxidematerials making high temperature rapid thermal oxidation (RTO)processes necessary to repair such damage while leading to theundesirable effect of crystallization of an amorphous high-k dielectricfilm. Proposed solutions to improve processing condition for forminghigh-k gate dielectrics with acceptable electrical properties, such ascapacitance and leakage current, have included efforts to improve thethermal stability of the high-k dielectric films thereby avoiding filmcrystallization, or to provide processes whereby lower processtemperatures (lower thermal budgets) are achieved, which have met withlimited success.

Therefore it would be advantageous to develop a reliable method andstructure for forming high-k gate dielectrics with associated gateelectrodes that overcome the shortcomings of the prior art includingimproved electrical properties.

It is therefore an object of the invention to provide a reliable methodfor forming high-k gate dielectrics with associated gate electrodes thatovercome the shortcomings of the prior art including improved electricalproperties.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the present invention provides a method for forming a gate stackhaving improved electrical properties in a gate structure formingprocess.

In a first embodiment, the method includes providing a semiconductorsubstrate; forming a metal oxide layer over an exposed portion of thesemiconductor substrate; and, forming a layer of polysilicon over themetal oxide layer according to an LPCVD process comprising a nitrogencontaining ambient.

In another embodiment, the method forms an improved high dielectricconstant gate structure including an interfacial layer disposed over andcontacting a silicon substrate having a thickness of less than about 10Angstroms; a high dielectric constant metal oxide layer disposed overand contacting the interfacial layer having a dielectric constant ofgreater than about 10; a barrier layer disposed over and contacting thehigh dielectric constant metal oxide layer; and, an electrode layercomprising polysilicon disposed over and contacting the barrier layer.

These and other embodiments, aspects and features of the invention willbe better understood from a detailed description of the preferredembodiments of the invention which are further described below inconjunction with the accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side view of an exemplary CMOS transistoraccording to an exemplary implementation of an embodiment of the presentinvention.

FIGS. 2A–2B are cross sectional side views of a portion of a gatestructure showing exemplary embodiments of the present invention.

FIGS. 3A and 3B are electrical measurement data showing the performanceof an exemplary device formed according to an embodiment of the presentinvention contrasted with device formed according to alternativemethods.

FIGS. 4A and 4B are electrical measurement data demonstrating theperformance of exemplary devices formed according to an embodiment ofthe present invention.

FIG. 5 is electrical measurement data showing contrasting devices formedaccording to preferred embodiments of the invention compared to priorart devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the method of the present invention is explained with referenceto the use of exemplary high-k gate dielectrics it will be appreciatedthat the method of the present invention may be adapted for the use ofany high-k material in the formation of a gate dielectric. By the termhigh-k dielectric is meant a material that has a dielectric constant ofgreater than about 10. The term “substrate” is defined to mean anysemiconductive substrate material including conventional semiconductorwafers.

Referring to FIG. 1 is shown a cross sectional side view of an exemplaryCMOS transistor having a high-k dielectric gate structure 18A includinga gate stack according to an embodiment of the present invention. Shownis a semiconductor substrate 12, for example a silicon substrateincluding lightly doped regions e.g., 14A, source/drain regions, e.g.,14B and shallow trench isolation regions, e.g., 16 formed in the siliconsubstrate by conventional methods known in the art. The regions 14A and14B are typically formed following the formation of the gate structureby ion implantation and annealing processes known in the art. Typicallythe ion implantation process is carried out following formation of thegate structure 18A including the gate dielectric region 18B formed ofmultiple layers including for example, an interfacial silicon dioxidelayer 18C, and gate electrode portion 18D, for example polysilicon. Thegate structure is typically formed by conventional photolithographicpatterning and anisotropic etching steps following polysilicondeposition. Following gate structure formation a first ion implantationprocess is carried out to form LDD regions e.g., 14A in the siliconsubstrate. Sidewall spacers e.g., 20A, are formed including for exampleat least one of silicon oxide (e.g., SiO₂), silicon oxynitride (e.g.,SiON), and silicon nitride (e.g., SiN) including multiple layeredspacers by methods known in the art including conventional depositionand etchback processes. A second ion implantation process is thencarried out to form the source/drain regions e.g., 14B in a self alignedion implantation process where the sidewall spacers e.g., 20A act as animplantation mask to form N type or P type doping regions depending onwhether a PMOS or NMOS type device is desired.

Referring to FIG. 2A is shown a cross sectional side view of a portionof the gate stack region e.g., 18A in FIG. 1. In an exemplary embodimentof the present invention, in a first step the silicon substrate 20 iscleaned prior to the optional formation of an overlying interfaciallayer 22. In one embodiment, the silicon substrate is cleaned usingstandard cleaning 1 (SC-1) and/or standard cleaning-2 (SC-2) solutions,which may be individually or sequentially used cleaning solutionsincluding mixtures of NH₄OH—H₂O₂—H₂O, and HCl—H₂O₂—H₂O, respectively.

In one embodiment of the invention an interfacial layer 22 is providedover the silicon substrate 20, formed of, for example one of silicondioxide (SiO₂), silicon oxynitride (e.g., SiON), silicon nitride (e.g.,SiN), and aluminum oxide (e.g., Al₂O₃). The interfacial layer 22 ispreferably formed to a thickness of less than about 10 Angstroms overthe silicon substrate and may be formed by conventional methodsincluding thermal oxidation, CVD, and nitridation, including plasmaassisted oxidation, CVD, and nitridation. For example in one embodiment,an SiO₂ interfacial layer, the SiO₂ layer is formed by exposure of thesilicon substrate to an oxygen atmosphere to form a native oxidefollowing the silicon substrate cleaning process.

A high-k dielectric layer 24 is deposited over the silicon substrateafter the cleaning process or the optionally formed interfacial layer ifprovided. The high-k dielectric material preferably has a dielectricconstant of greater than about 10. For example, the high-k dielectriclayer is preferably formed of a metal oxide including tantalum oxides(e.g., Ta₂O₅), titanium oxides, (e.g., TiO₂), hafnium oxides (e.g.,HfO₂), yttrium oxides (e.g., Y₂O₃), and lanthanum oxides (e.g., La₂O₅).The high-k dielectric layer 24 is preferably formed by one of metalorganic CVD (MOCVD) and atomic layer deposition (ALD) or epitaxy. Otherless preferably processes including plasma enhanced CVD, evaporationdeposition processes, and reactive sputtering deposition processes maybe used. The high-k dielectric layer 24 is preferably formed having athickness of between about 20 Angstroms to about 100 Angstroms.

For example in a MOCVD process, a liquid metal-organic precursor isvaporized and provided over a process wafer substrate at a pressure ofabout 0.25 Torr to about 10 Torr, more preferably about 100 mTorr toabout 500 mTorr, to dissociate on the substrate surface in the presenceoxygen to form the high-k metal oxide. The MOCVD deposition processpreferably takes place with the wafer substrate heated to about 350° C.to about 450° C. In the ALD process which gives superior film qualities,although a slower process, molecular layers are sequentially depositedincluding a molecular layer of metal precursor, for example ametal-organic precursor, followed by controlled dissociation andoxidation of the metal-organic molecular layer to form a portion of thehigh-k dielectric layer, the process being sequentially repeated tocomplete the formation of the high-k dielectric layer. The high-kdielectric layers are optionally annealed in an oxygen containingatmosphere from about 400° C. to about 800° C. following deposition toimprove the oxide quality and dielectric properties. In a preferredembodiment, the high-k dielectric layers (metal oxide layers) areannealed in a mixture of ozone and oxygen, the mixture having avolumetric ratio of oxygen to ozone of about 1 to 1 to about 10 to 1with respect to the mixture volume at a temperature of about 450° C. toabout 550° C. For example, the lower temperature anneal in oxygenminimize the formation or increasing thickness of an interfacial siliconoxide layer which lowers the overall gate stack capacitance.

Following deposition of the high-k dielectric layer, a polysilicon layer26 is preferably deposited by a low pressure CVD (LPCVD) processpreferably using silane gas (SiH₄) as the reactive gas and nitrogen asthe carrier gas for a nitrogen ambient deposition process. Preferably,the polysilicon layer 26 is deposited having a thickness of about 1500Angstroms to about 2000 Angstroms. For example the CVD precursor gas ispreferably about 15 volume percent to about 40 volume percent silanewith respect to the total volume of the CVD precursor gas with theremaining volume percent portion nitrogen gas. For example the LPCVDprocess is preferably carried out at a pressure of about 100 milliTorrto about 1 Torr. In addition, the LPCVD process may include a cold-wallor hot-wall configuration. Preferably, the substrate temperature ismaintained at about 250° C. to about 450° C.

Referring to FIG. 2B, in another embodiment, the gate stack is formed inthe same manner as explained with respect to FIG. 2A except that a metaloxide barrier layer, for example aluminum oxide (e.g., Al₂O₃) 24B isformed over the high-k dielectric layer 24 prior to the formation of thepolysilicon layer 26. The barrier layer 24B prevents interaction of thehigh-k dielectric layer 24 with the polysilicon layer 26. As with thehigh-k dielectric layer 24, the aluminum oxide barrier layer ispreferably formed by metal organic CVD (MOCVD), or atomic layerdeposition (ALD) process, although other conventional depositionprocesses may be used including evaporation deposition processes andreactive sputtering deposition processes. Preferably, the aluminum oxidebarrier layer 24B is formed having a thickness of less than about 15Angstroms, more preferably about 5 to about 10 Angstroms. Followingformation of the aluminum oxide barrier layer a polysilicon layer forforming the gate electrode is optionally deposited according to thepreferred embodiments discussed with respect to FIG. 2A. However,additional polysilicon depositions are now suitably used including oneof an LPCVD and APCVD deposition processes carried out in an ambientincluding at least one of nitrogen and hydrogen.

In an exemplary embodiment of the present invention, a hafnium oxidelayer (e.g., HfO₂) was used as the high-k gate dielectric layer in agate stack. According to an aspect of the invention, including carryingout the polysilicon layer deposition according to an LPCVD process in anitrogen ambient according to preferred embodiments, it was found thatthe electrical properties including capacitance versus voltage and gateleakage current were significantly improved compared to, for example,CVD depositions of polysilicon according to atmospheric pressure CVD(APCVD) methods in either a nitrogen or hydrogen ambient.

For example referring to FIG. 3A, are shown capacitance values on thevertical axis and applied voltage on the horizontal axis in aconventional CV (capacitance-voltage) test. Lines B1, C1 and D1represent respectively gates structure samples B1, C1, and D1, formedwith polysilicon layer depositions carried out over the hafnium oxidehigh-k dielectric layer respectively using APCVD in a nitrogen ambient(B1), APCVD in a hydrogen ambient (C1), and LPCVD in a hydrogen ambient(D1). In contrast, line A1 represents the CV characteristics of sampleA1 where a gate stack is formed according to a preferred embodiment ofthe present invention including a hafnium oxide (e.g., HfO₂) high-kdielectric layer and polysilicon electrode layer deposited according anLPCVD polysilicon deposition process in a nitrogen containing ambient.It is apparent that the electrical properties including CVcharacteristics of the gate structure formed according to preferredembodiments are improved compared to alternative processes includingprior art processes.

For example, referring to FIG. 3B are shown data from conventionalcurrent leakage tests performed on the same set of test samples A1, B1,C1, and D1, discussed with respect to FIG. 3A. The gate current(including leakage current) is shown on the vertical axis and theapplied gate voltage is shown on the horizontal axis. With respect tosamples B1, C1, and D1 the data e.g., lines B1, C1, and D1 respectively,shows about the same gate current with applied gate voltage. Incontrast, sample A1 gate current as shown by line A, shows lower gatecurrent (including lower leakage current) at a given applied voltagecompared to polysilicon forming processes represented by samples B, C,and D. It is apparent that the electrical properties including gatecurrent characteristics of the gate structure formed according topreferred embodiments are improved compared to alternative processesincluding prior art processes.

Referring to FIGS. 4A and 4B, the CV tests and gate current tests areshown for another embodiment of the invention where an aluminum oxidecapping layer having a thickness of from about 5 Angstroms to about 15Angstroms, preferably less than about 10 Angstroms is provided over thehigh-k dielectric layer prior to polysilicon deposition. The conditionsfor deposition of polysilicon as shown for samples A2, B2, C2, D2, arethe same as for samples A1, B1, C1, and D1 as discussed with respect toFIGS. 3A and 3B. Although, hard to distinguish on the data shown, allsamples A2, B2, C2, D2 have similar CV values as shown in FIGS. 4A and4B. Acceptable electrical properties are now obtained for both APCVD andLPCVD deposition processes carried out in both nitrogen containingand/or hydrogen containing ambients. Thus, the addition of the aluminumoxide barrier layer according to an embodiment of the present inventionadvantageously provides protection of the high-k dielectric layer fromthe effects of polysilicon deposition processing conditions andincreases the deposition options for depositing the polysiliconelectrode.

Referring to FIG. 5, is shown data where gate current is represented onthe vertical axis and an equivalent dielectric thickness with respect toa silicon dioxide gate dielectric is represented on the horizontal axis.Line 1 represents values of gate current as a function of SiO₂ gatedielectric physical thicknesses. Also shown are representative gatecurrent values for samples A1, A2, B2, C2, and D2 formed according topreferred embodiments of the present invention, shown representing adielectric thickness equivalent to a silicon dioxide thickness(equivalent SiO₂ dielectric thickness) of between about 10 Angstroms andabout 20 Angstroms, more precisely, about 15 Angstroms. It can be seenthat according to devices produced according to preferred embodiments ofthe present invention the gate current for samples A1, A2, B2, C2, andD2 is improved (e.g., lower leakage current) about 4 to about 5 ordersof magnitude at about an equivalent SiO₂ dielectric thickness (EOT) ofabout 15 Angstroms.

While the embodiments illustrated in the Figures and described above arepresently preferred, it should be understood that these embodiments areoffered by way of example only. The invention is not limited to aparticular embodiment, but extends to various modifications,combinations, and permutations as will occur to the ordinarily skilledartisan that nevertheless fall within the scope of the appended claims.

1. A method for forming a gate stack having improved electricalproperties in a gate structure forming process comprising the steps of:providing a semiconductor substrate; forming a metal oxide layer over anexposed portion of the semiconductor substrate; and forming asilicon-containing electrode layer over the metal oxide layer bydepositing a silicon layer and introducing a nitrogen containing ambientsimultaneously.
 2. The method of claim 1, wherein the metal oxide layercomprises a dielectric constant of greater than about
 20. 3. The methodof claim 1, wherein the metal oxide is formed having a thickness ofabout 20 Angstroms to about 100 Angstroms.
 4. The method of claim 1,wherein the gate stack including the metal oxide layer is formed to havea dielectric thickness equivalent to a silicon dioxide dielectricthickness of less than about 20 Angstroms.
 5. The method of claim 3,wherein the metal oxide is selected from the group consisting oftantalum oxides, titanium oxides, zirconium oxides, hafnium oxides, andyttrium oxides.
 6. The method of claim 5, wherein the metal oxide isformed from one of a metal-organic CVD method and an atomic layerdeposition (ALD) method.
 7. The method of claim 6, wherein an ozonecontaining oxidation process is carried out to treat the metal oxidelayer following the formation of the metal oxide layer.
 8. The method ofclaim 1, wherein a layer comprising aluminum oxide is formed over themetal oxide layer.
 9. The method of claim 8, wherein the aluminum oxidelayer is formed having a thickness of about 5 Angstroms to about 15Angstroms.
 10. A method for forming a gate stack having improvedelectrical properties in a gate structure forming process comprising thesteps of: providing a semiconductor substrate; forming a metal oxidelayer over an exposed portion of the semiconductor substrate; anddepositing a polysilicon electrode over the metal oxide layer by a lowpressure chemical vapor deposition (LPCVD) process simultaneously inassociation with a nitrogen containing ambient.
 11. The method of claim10, wherein the LPCVD process comprises a cold-wall and a hot-wallconfiguration.